Housing
What size is your tank? 2 gallons
What temperature is your tank? 78 degrees F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Omega One Betta Pellets
How often do you feed your betta fish? 1-2 per day

Maintenance
How often do you perform a water change? Once every 2 weeks
What percentage of the water do you change when you perform a water change? 25%

When did you start noticing the symptoms? One day, he started darting around the tank and stopped. On occasion, he will dart around.
Have you started treating your fish? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? 3 months

Housing
What size is your tank? 2 gallons
What temperature is your tank? 78 degrees F
Does your tank have a filter? Yes
Does your tank have an air stone or other type of aeration? No
Is your tank heated? Yes
What tank mates does your betta fish live with? None

Food
What type of food do you feed your betta fish? Omega One Betta Pellets
How often do you feed your betta fish? 1-2 per day

Maintenance
How often do you perform a water change? Once every 2 weeks
What percentage of the water do you change when you perform a water change? 25%

When did you start noticing the symptoms? One day, he started darting around the tank and stopped. On occasion, he will dart around.
Have you started treating your fish? No
Does your fish have any history of being ill? No
How old is your fish (approximately)? 3 months

How often do you perform a water change? Once every 2 weeks
What percentage of the water do you change when you perform a water change? 25%

^ This is why your fish is dying. He's darting around because his tank is full of toxins and it's very likely he has ammonia poisoning, from all the built up pee in his water.

Put him in a covered cup with a little old tank water. Change 100% of his water and rinse the tank out ... now-ish? And then refill it with warm water as close to his usual temp as possible.

If you have any rock, kosher or aquarium salt, add 1 teaspoon (pre-dissolve it, make sure it's all dissolved before he goes in it) per gallon. Do this just for this one water change, it'll help him with the ammonia poisoning. If you don't have this, don't worry. Just clean water will do.

Because he needs some clean water, fast.

Once you've got the clean water in and conditioned it, float his little cup in the tank to help acclimate him (get him used to the temp) and add a little new water to the old every few mins.

Tomorrow, do another 100% change - no salt, this time.

Please, please change your water way more often? This will really help your fish remain healthy..

ETA: If you can, purchase a product called Stress Coat. This will help him recover. It helps to heal the slime coat, which is what keeps a fish healthy. Ammonia erodes this, and makes the fish more likely to get ill.

You'll need an antibiotic like Kanaplex, to hit it fast. It can kill fish pretty quickly, or it can kill them over a few weeks. One of mine had it for two months before he died of it (it IS immensely curable, but he was sickly, and I just could not shift it).

So I don't mess about once I see fuzzy patches. Into antibiotic they go.

This is another good reason to keep your water quality up. Nice looking tank, btw, and glad to hear he's picked up a bit.

Is that a Dymax tank he's in?

ETAL Maracyn 2 (NOT Maracyn 1) and Furan 2 are both also recommended, as Kanaplex is apparently somewhat hard to find in stores (which is odd, as many US fishkeepers swear by it)..

You'll need an antibiotic like Kanaplex, to hit it fast. It can kill fish pretty quickly, or it can kill them over a few weeks. One of mine had it for two months before he died of it (it IS immensely curable, but he was sickly, and I just could not shift it).

So I don't mess about once I see fuzzy patches. Into antibiotic they go.

This is another good reason to keep your water quality up. Nice looking tank, btw, and glad to hear he's picked up a bit.